The invention relates to a method for the preparation of thin films of high-temperature-resistant metals such as tungsten, molybdenum, rhenium or osmium on a heated high-temperature-resistant substrate by thermal evaporation of the oxides of the high-temperature resistant metals in a high vacuum.
Heavy metals such as tungsten, molybdenum, rhenium or osmium find application in electron and X-ray tubes and more recently also in highly integrated (LSI) circuits in solid-state electronics, because of their high melting point and their high-temperature strength. Their low tendency to weld makes it possible to utilize them in switch contacts and in addition, such metals are currently being considered and tested as protective layers for structural parts of fusion reactors.
Most of the mentioned applications, especially in X-ray anodes, integrated circuits, contacts and in casting structural parts, require for economic or functional reasons, thin films of such high-temperature-resistant metals on substrates which can be subjected to relatively high thermal stresses.
The preparation of these thin films can be accomplished by expensive and laborious methods such as fusion electrolysis, precipitation from the gaseous phase from chemical compounds or by plasma spraying. Vapor deposition of the thin films on the substrates, however, presents difficulties because of the high melting point and the low vapor pressure of metals such as tungsten, molybdenum, rhenium or osmium. Thus, while it is possible to vapor-deposit tungsten or molybdenum by means of electron beam evaporation or by cathode sputtering in a vacuum, thermal evaporation in a high vacuum, which is far more economical and easier to control insofar as the formation of layers is concerned, fails mainly because of the high evaporation temperatures.
From the German Pat. No. 2,346,394, a method for the preparation of thin films of tungsten or molybdenum on a high-temperature-resistant substrate by thermal evaporation in a high vacuum is known. This known method leads to heavy-metal layers via the evaporation of the oxides of tungsten and molybdenum and decomposition at a hot substrate.